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kps98
Associate III
November 6, 2024
Solved

VL53L4CD number of spad used

  • November 6, 2024
  • 1 reply
  • 1194 views

Hi,

I noticed that the number of spad used reduces as an object gets closer to VL53L4CD. I was thinking about using this to make my distance measurement better. It also helps me to detect if the object is moving much in the field of view as i noticed, once i move away from the field of view of sensor number of spad increases again. But this behaviour can only be observed below 20cm as after that the number of spad peaks. Is there anyway i can reconfigure the sensor to give me similar behaviour till 40cm?

I realise that reducing the number of active SPADs could reduce accuracy, but I'm wondering if there’s a way to reconfigure the sensor to maintain a similar pattern at greater distances. I just want to test and see if i could find something interesting out of it.

Regards,

Kiran

Best answer by John E KVAM

The sensor really wants to see 20Million photons per second. More does not help, and it costs power to reset a SPAD after it triggers. 

The light intensity spreads as a square of the distance. 

So near objects reflect a LOT more photons than a far object. And White reflects a lot more than black.

To get the desired signal we adjust the number of Single Photon Avalanche Diodes (SPAD) photon detectors. 

So, you will be using 3-4 SPADS with a white paper at a close distance. And your signal rate will still be huge. 

But with a hand a 50cm, you will be using nearly 200 SPADs and still only getting a signal rate of less than 1Million photons per second. 

Absolute signal rate doesn't do you much good. Unless you consider how many SPADS we are using. 

You can get a signal rate of 20M using 4 SPADS, and the same 20M using 150 SPAD, but the reflectivity of the object is quite different. 

I tried using Signal Per SPAD - normalized assuming 200 SPADS (Signal/SPAD *200) as a measure of reflectivity. 

And you might consider using that. 

(There are really 256 SPADS, but some are occluded to handle the really high signal cases, so 200 is about right. It's also why the SPAD count is not an integer.)

 

 

 

1 reply

John E KVAM
John E KVAMBest answer
ST Employee
November 6, 2024

The sensor really wants to see 20Million photons per second. More does not help, and it costs power to reset a SPAD after it triggers. 

The light intensity spreads as a square of the distance. 

So near objects reflect a LOT more photons than a far object. And White reflects a lot more than black.

To get the desired signal we adjust the number of Single Photon Avalanche Diodes (SPAD) photon detectors. 

So, you will be using 3-4 SPADS with a white paper at a close distance. And your signal rate will still be huge. 

But with a hand a 50cm, you will be using nearly 200 SPADs and still only getting a signal rate of less than 1Million photons per second. 

Absolute signal rate doesn't do you much good. Unless you consider how many SPADS we are using. 

You can get a signal rate of 20M using 4 SPADS, and the same 20M using 150 SPAD, but the reflectivity of the object is quite different. 

I tried using Signal Per SPAD - normalized assuming 200 SPADS (Signal/SPAD *200) as a measure of reflectivity. 

And you might consider using that. 

(There are really 256 SPADS, but some are occluded to handle the really high signal cases, so 200 is about right. It's also why the SPAD count is not an integer.)

 

 

 

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